RFC 5587 PROPOSED STANDARD GSS-API · SECURITY · API
GSS-API mech inquiry: Extended Generic Security Service Mechanism Inquiry APIs
New GSS-API calls that let applications pick a security mechanism by what it can do instead of hardcoding its identifier.
RFC EDITOR
Williams, N., "Extended Generic Security Service Mechanism Inquiry APIs", RFC 5587, DOI 10.17487/RFC5587, July 2009, <https://www.rfc-editor.org/info/rfc5587>.
BIBTEX
@misc{rfc5587,
series = {Request for Comments},
number = 5587,
howpublished = {RFC 5587},
publisher = {RFC Editor},
doi = {10.17487/RFC5587},
url = {https://www.rfc-editor.org/info/rfc5587},
author = {N. Williams},
title = {{Extended Generic Security Service Mechanism Inquiry APIs}},
pagetotal = 16,
year = 2009,
month = jul,
} RFC text
Contents
Network Working Group N. Williams
Request for Comments: 5587 Sun
Category: Standards Track July 2009
Extended Generic Security Service Mechanism Inquiry APIs
Abstract
This document introduces new application programming interfaces
(APIs) to the Generic Security Services API (GSS-API) for extended
mechanism attribute inquiry. These interfaces are primarily intended
to reduce instances of hardcoding of mechanism identifiers in GSS
applications.
These interfaces include mechanism attributes and attribute sets, a
function for inquiring the attributes of a mechanism, a function for
indicating mechanisms that possess given attributes, and a function
for displaying mechanism attributes.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. New GSS-API Interfaces ..........................................3
3.1. Mechanism Attributes and Attribute Sets ....................3
3.2. List of Known Mechanism Attributes .........................4
3.3. Mechanism Attribute Sets of Existing Mechs .................6
3.4. New GSS-API Function Interfaces ............................8
3.4.1. Mechanism Attribute Criticality .....................8
3.4.2. GSS_Indicate_mechs_by_attrs() .......................9
3.4.3. GSS_Inquire_attrs_for_mech() .......................10
3.4.4. GSS_Display_mech_attr() ............................10
3.4.5. New Major Status Values ............................11
3.4.6. C-Bindings .........................................11
4. Requirements for Mechanism Designers ...........................13
5. IANA Considerations ............................................13
6. Security Considerations ........................................13
7. References .....................................................13
7.1. Normative References ......................................13
7.2. Informative References ....................................14
Appendix A. Typedefs and C Bindings ..................................15
1. Introduction
GSS-API [RFC2743] mechanisms have a number of properties that may be
of interest to applications. The lack of APIs for inquiring about
available mechanisms' properties has meant that many GSS-API
applications must hardcode mechanism Object Identifiers (OIDs).
Ongoing work may result in a variety of new GSS-API mechanisms.
Applications should not have to hardcode their OIDs.
For example, the Secure Shell version 2 (SSHv2) protocol [RFC4251]
supports the use of GSS-API mechanisms for authentication [RFC4462]
but explicitly prohibits the use of Simple and Protected GSS-API
Negotiation (SPNEGO) [RFC4178]. Future mechanisms that negotiate
mechanisms would have to be forbidden as well, but there is no way to
implement applications that inquire what mechanisms are available and
then programmatically exclude mechanisms "like SPNEGO".
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. New GSS-API Interfaces
We introduce a new concept -- that of mechanism attributes. By
allowing applications to query the set of attributes associated with
individual mechanisms and to find out which mechanisms support a
given set of attributes, we allow applications to select mechanisms
based on their attributes without having to hardcode mechanism OIDs.
Section 3.1 describes the mechanism attributes concept. Sections
3.4.2, 3.4.3, and 3.4.4 describe three new interfaces that deal in
mechanisms and attribute sets:
o GSS_Indicate_mechs_by_attrs()
o GSS_Inquire_attrs_for_mech()
o GSS_Display_mech_attr()
3.1. Mechanism Attributes and Attribute Sets
An abstraction for the features provided by mechanisms and pseudo-
mechanisms is needed in order to facilitate the programmatic
selection of mechanisms. Pseudo-mechanisms are mechanisms that make
reference to other mechanisms in order to provide their services.
For example, SPNEGO is a pseudo-mechanism, for without other
mechanisms SPNEGO is useless.
Two data types are needed: one for individual mechanism attributes
and one for mechanism attribute sets. To simplify the mechanism
attribute interfaces, we reuse the 'OID' and 'OID set' data types and
model individual mechanism attribute types as OIDs.
To this end, we define an open namespace of mechanism attributes and
assign them arcs off of this OID:
<1.3.6.1.5.5.13>
Each mechanism has a set of mechanism attributes that it supports as
described in its specification.
3.2. List of Known Mechanism Attributes
+-------------------------+---------+-------------------------+
| Mech Attr Name | OID Arc | Arc Name |
+-------------------------+---------+-------------------------+
| GSS_C_MA_MECH_CONCRETE | (1) | concrete-mech |
| GSS_C_MA_MECH_PSEUDO | (2) | pseudo-mech |
| GSS_C_MA_MECH_COMPOSITE | (3) | composite-mech |
| GSS_C_MA_MECH_NEGO | (4) | mech-negotiation-mech |
| GSS_C_MA_MECH_GLUE | (5) | mech-glue |
| GSS_C_MA_NOT_MECH | (6) | not-mech |
| GSS_C_MA_DEPRECATED | (7) | mech-deprecated |
| GSS_C_MA_NOT_DFLT_MECH | (8) | mech-not-default |
| GSS_C_MA_ITOK_FRAMED | (9) | initial-is-framed |
| GSS_C_MA_AUTH_INIT | (10) | auth-init-princ |
| GSS_C_MA_AUTH_TARG | (11) | auth-targ-princ |
| GSS_C_MA_AUTH_INIT_INIT | (12) | auth-init-princ-initial |
| GSS_C_MA_AUTH_TARG_INIT | (13) | auth-targ-princ-initial |
| GSS_C_MA_AUTH_INIT_ANON | (14) | auth-init-princ-anon |
| GSS_C_MA_AUTH_TARG_ANON | (15) | auth-targ-princ-anon |
| GSS_C_MA_DELEG_CRED | (16) | deleg-cred |
| GSS_C_MA_INTEG_PROT | (17) | integ-prot |
| GSS_C_MA_CONF_PROT | (18) | conf-prot |
| GSS_C_MA_MIC | (19) | mic |
| GSS_C_MA_WRAP | (20) | wrap |
| GSS_C_MA_PROT_READY | (21) | prot-ready |
| GSS_C_MA_REPLAY_DET | (22) | replay-detection |
| GSS_C_MA_OOS_DET | (23) | oos-detection |
| GSS_C_MA_CBINDINGS | (24) | channel-bindings |
| GSS_C_MA_PFS | (25) | pfs |
| GSS_C_MA_COMPRESS | (26) | compress |
| GSS_C_MA_CTX_TRANS | (27) | context-transfer |
| <reserved> | (28...) | |
+-------------------------+---------+-------------------------+
Table 1
+-------------------------+-----------------------------------------+
| Mech Attr Name | Purpose |
+-------------------------+-----------------------------------------+
| GSS_C_MA_MECH_CONCRETE | Indicates that a mech is neither a |
| | pseudo-mechanism nor a composite |
| | mechanism. |
| GSS_C_MA_MECH_PSEUDO | Indicates that a mech is a |
| | pseudo-mechanism. |
| GSS_C_MA_MECH_COMPOSITE | Indicates that a mech is a composite of |
| | other mechanisms. This is reserved for |
| | a specification of "stackable" |
| | pseudo-mechanisms. |
| GSS_C_MA_MECH_NEGO | Indicates that a mech negotiates other |
| | mechs (e.g., SPNEGO has this |
| | attribute). |
| GSS_C_MA_MECH_GLUE | Indicates that the OID is not for a |
| | mechanism but for the GSS-API itself. |
| GSS_C_MA_NOT_MECH | Indicates that the OID is known, yet it |
| | is also known not to be the OID of any |
| | GSS-API mechanism (or of the GSS-API |
| | itself). |
| GSS_C_MA_DEPRECATED | Indicates that a mech (or its OID) is |
| | deprecated and MUST NOT be used as a |
| | default mechanism. |
| GSS_C_MA_NOT_DFLT_MECH | Indicates that a mech (or its OID) MUST |
| | NOT be used as a default mechanism. |
| GSS_C_MA_ITOK_FRAMED | Indicates that the given mechanism's |
| | initial context tokens are properly |
| | framed as per Section 3.1 of [RFC2743]. |
| GSS_C_MA_AUTH_INIT | Indicates support for authentication of |
| | initiator to acceptor. |
| GSS_C_MA_AUTH_TARG | Indicates support for authentication of |
| | acceptor to initiator. |
| GSS_C_MA_AUTH_INIT_INIT | Indicates support for "initial" |
| | authentication of initiator to |
| | acceptor. "Initial authentication" |
| | refers to the use of passwords, or keys |
| | stored on tokens, for authentication. |
| | Whether a mechanism supports initial |
| | authentication may depend on IETF |
| | consensus (see Security |
| | Considerations). |
| GSS_C_MA_AUTH_TARG_INIT | Indicates support for initial |
| | authentication of acceptor to |
| | initiator. |
| GSS_C_MA_AUTH_INIT_ANON | Indicates support for |
| | GSS_C_NT_ANONYMOUS as an initiator |
| | principal name. |
| GSS_C_MA_AUTH_TARG_ANON | Indicates support for |
| | GSS_C_NT_ANONYMOUS as a target |
| | principal name. |
| GSS_C_MA_DELEG_CRED | Indicates support for credential |
| | delegation. |
| GSS_C_MA_INTEG_PROT | Indicates support for per-message |
| | integrity protection. |
| GSS_C_MA_CONF_PROT | Indicates support for per-message |
| | confidentiality protection. |
| GSS_C_MA_MIC | Indicates support for Message Integrity |
| | Code (MIC) tokens. |
| GSS_C_MA_WRAP | Indicates support for WRAP tokens. |
| GSS_C_MA_PROT_READY | Indicates support for per-message |
| | protection prior to full context |
| | establishment. |
| GSS_C_MA_REPLAY_DET | Indicates support for replay detection. |
| GSS_C_MA_OOS_DET | Indicates support for out-of-sequence |
| | detection. |
| GSS_C_MA_CBINDINGS | Indicates support for channel bindings. |
| GSS_C_MA_PFS | Indicates support for Perfect Forward |
| | Security. |
| GSS_C_MA_COMPRESS | Indicates support for compression of |
| | data inputs to GSS_Wrap(). |
| GSS_C_MA_CTX_TRANS | Indicates support for security context |
| | export/import. |
+-------------------------+-----------------------------------------+
Table 2
3.3. Mechanism Attribute Sets of Existing Mechs
The Kerberos V mechanism [RFC1964] provides the following mechanism
attributes:
o GSS_C_MA_MECH_CONCRETE
o GSS_C_MA_ITOK_FRAMED
o GSS_C_MA_AUTH_INIT
o GSS_C_MA_AUTH_TARG
o GSS_C_MA_DELEG_CRED
o GSS_C_MA_INTEG_PROT
o GSS_C_MA_CONF_PROT
o GSS_C_MA_MIC
o GSS_C_MA_WRAP
o GSS_C_MA_PROT_READY
o GSS_C_MA_REPLAY_DET
o GSS_C_MA_OOS_DET
o GSS_C_MA_CBINDINGS
o GSS_C_MA_CTX_TRANS (some implementations, using implementation-
specific exported context token formats)
The Kerberos V mechanism also has a deprecated OID that has the same
mechanism attributes as above as well as GSS_C_MA_DEPRECATED.
The mechanism attributes of the Simple Public-Key GSS-API Mechanism
(SPKM) [RFC2025] family of mechanisms will be provided in a separate
document, as SPKM is currently being reviewed for possibly
significant changes due to problems in its specifications.
The Low Infrastructure Public Key (LIPKEY) mechanism [RFC2847] offers
the following attributes:
o GSS_C_MA_MECH_CONCRETE
o GSS_C_MA_ITOK_FRAMED
o GSS_C_MA_AUTH_INIT_INIT
o GSS_C_MA_AUTH_TARG (from SPKM-3)
o GSS_C_MA_AUTH_TARG_ANON (from SPKM-3)
o GSS_C_MA_INTEG_PROT
o GSS_C_MA_CONF_PROT
o GSS_C_MA_REPLAY_DET
o GSS_C_MA_OOS_DET
o GSS_C_MA_CTX_TRANS (some implementations, using implementation-
specific exported context token formats)
(LIPKEY should also provide GSS_C_MA_CBINDINGS, but SPKM-3
requires clarifications on this point.)
The SPNEGO mechanism [RFC4178] provides the following attributes:
o GSS_C_MA_MECH_NEGO
o GSS_C_MA_ITOK_FRAMED
All other mechanisms' attributes will be described elsewhere.
3.4. New GSS-API Function Interfaces
Several new interfaces are given by which, for example, GSS-API
applications may determine what features are provided by a given
mechanism and what mechanisms provide what features.
These new interfaces are all OPTIONAL.
Applications should use GSS_Indicate_mechs_by_attrs() instead of
GSS_Indicate_mechs() wherever possible.
Applications can use GSS_Indicate_mechs_by_attrs() to determine what,
if any, mechanisms provide a given set of features.
GSS_Indicate_mechs_by_attrs() can also be used to indicate (as in
GSS_Indicate_mechs()) the set of available mechanisms of each type
(concrete, mechanism negotiation pseudo-mechanism, etc.).
3.4.1. Mechanism Attribute Criticality
Mechanism attributes may be added at any time. Not only may
attributes be added to the list of known mechanism attributes at any
time, but the set of mechanism attributes supported by a mechanism
can be changed at any time.
For example, new attributes might be added to reflect whether a
mechanism's initiator must contact an online infrastructure and/or
whether the acceptor must do so. In this example, the Kerberos V
mechanism would gain a new attribute even though the mechanism itself
is not modified.
Applications making use of attributes not defined herein would then
have no way of knowing whether a GSS-API implementation and its
mechanisms know about new mechanism attributes. To address this
problem, GSS_Indicate_mechs_by_attrs() and
GSS_Inquire_attrs_for_mech() support a notion of critical mechanism
attributes. Applications can search for mechanisms that understand
mechanism attributes that are critical to the application, and the
application may ask what mechanism attributes are understood by a
given mechanism.
3.4.2. GSS_Indicate_mechs_by_attrs()
Inputs:
o desired_mech_attrs SET OF OBJECT IDENTIFIER -- set of GSS_C_MA_*
OIDs that the mechanisms indicated in the mechs output parameter
MUST offer.
o except_mech_attrs SET OF OBJECT IDENTIFIER -- set of GSS_C_MA_*
OIDs that the mechanisms indicated in the mechs output parameter
MUST NOT offer.
o critical_mech_attrs SET OF OBJECT IDENTIFIER -- set of GSS_C_MA_*
OIDs that the mechanisms indicated in the mechs output parameter
MUST understand (i.e., mechs must know whether critical attributes
are or are not supported).
Outputs:
o major_status INTEGER
o minor_status INTEGER
o mechs SET OF OBJECT IDENTIFIER -- set of mechanisms that support
the given desired_mech_attrs but not the except_mech_attrs, and
all of which understand the given critical_mech_attrs (the caller
must release this output with GSS_Release_oid_set()).
Return major_status codes:
o GSS_S_COMPLETE indicates success; the output mechs parameter MAY
be the empty set (GSS_C_NO_OID_SET).
o GSS_S_FAILURE indicates that the request failed for some other
reason.
GSS_Indicate_mechs_by_attrs() returns the set of OIDs corresponding
to mechanisms that offer at least the desired_mech_attrs but none of
the except_mech_attrs, and that understand all of the attributes
listed in critical_mech_attrs.
When all three sets of OID input parameters are the empty set, this
function acts as a version of GSS_indicate_mechs() that outputs the
set of all supported mechanisms.
3.4.3. GSS_Inquire_attrs_for_mech()
Inputs:
o mech OBJECT IDENTIFIER -- mechanism OID
Outputs:
o major_status INTEGER
o minor_status INTEGER
o mech_attrs SET OF OBJECT IDENTIFIER -- set of mech_attrs OIDs
(GSS_C_MA_*) supported by the mechanism (the caller must release
this output with GSS_Release_oid_set()).
o known_mech_attrs SET OF OBJECT IDENTIFIER -- set of mech_attrs
OIDs known to the mechanism implementation (the caller must
release this output with GSS_Release_oid_set()).
Return major_status codes:
o GSS_S_COMPLETE indicates success; the output mech_attrs parameter
MAY be the empty set (GSS_C_NO_OID_SET).
o GSS_S_BAD_MECH indicates that the mechanism named by the mech
parameter does not exist or that the mech is GSS_C_NO_OID and no
default mechanism could be determined.
o GSS_S_FAILURE indicates that the request failed for some other
reason.
GSS_Inquire_attrs_for_mech() indicates the set of mechanism
attributes supported by a given mechanism.
3.4.4. GSS_Display_mech_attr()
Inputs:
o mech_attr OBJECT IDENTIFIER -- mechanism attribute OID
Outputs:
o major_status INTEGER
o minor_status INTEGER
o name OCTET STRING, -- name of mechanism attribute (e.g.,
GSS_C_MA_*).
o short_desc OCTET STRING, -- a short description of the mechanism
attribute (the caller must release this output with
GSS_Release_buffer()).
o long_desc OCTET STRING -- a longer description of the mechanism
attribute (the caller must release this output with
GSS_Release_buffer()).
Return major_status codes:
o GSS_S_COMPLETE indicates success.
o GSS_S_BAD_MECH_ATTR indicates that the mechanism attribute
referenced by the mech_attr parameter is unknown to the
implementation.
o GSS_S_FAILURE indicates that the request failed for some other
reason.
This function can be used to obtain human-readable descriptions of
GSS-API mechanism attributes.
3.4.5. New Major Status Values
A single, new, major status code is added for
GSS_Display_mech_attr():
o GSS_S_BAD_MECH_ATTR,
roughly corresponding to GSS_S_BAD_MECH but applicable to mechanism
attribute OIDs rather than to mechanism OIDs.
For the C-bindings of the GSS-API [RFC2744], GSS_S_BAD_MECH_ATTR
shall have a routine error number of 19 (this is shifted to the left
by GSS_C_ROUTINE_ERROR_OFFSET).
3.4.6. C-Bindings
Note that there is a bug in the C bindings of the GSS-APIv2u1
[RFC2744] in that the C 'const' attribute is applied to types that
are pointer typedefs. This is a bug because it declares that the
pointer argument is 'const' rather than that the object pointed by it
is const. To avoid this error, we hereby define new typedefs, which
include const properly:
typedef const gss_buffer_desc * gss_const_buffer_t;
typedef const struct gss_channel_bindings_struct *
gss_const_channel_bindings_t;
typedef const <platform-specific> gss_const_ctx_id_t;
typedef const <platform-specific> gss_const_cred_id_t;
typedef const <platform-specific> gss_const_name_t;
typedef const gss_OID_desc * gss_const_OID;
typedef const gss_OID_set_desc * gss_const_OID_set;
Figure 1: const typedefs
Note that only gss_const_OID and gss_const_OID_set are used below.
We include the other const typedefs for convenience since the C
bindings of the GSS-API do use const with pointer typedefs when it
should often instead use the above typedefs instead.
#define GSS_S_BAD_MECH_ATTR (19ul << GSS_C_ROUTINE_ERROR_OFFSET)
OM_uint32 gss_indicate_mechs_by_attrs(
OM_uint32 *minor_status,
gss_const_OID_set desired_mech_attrs,
gss_const_OID_set except_mech_attrs,
gss_const_OID_set critical_mech_attrs,
gss_OID_set *mechs);
OM_uint32 gss_inquire_attrs_for_mech(
OM_uint32 *minor_status,
gss_const_OID mech,
gss_OID_set *mech_attrs,
gss_OID_set *known_mech_attrs);
OM_uint32 gss_display_mech_attr(
OM_uint32 *minor_status,
gss_const_OID mech_attr,
gss_buffer_t name,
gss_buffer_t short_desc,
gss_buffer_t long_desc);
Figure 2: C bindings
Note that output buffers must be released via gss_release_buffer().
Output OID sets must be released via gss_release_oid_set().
Please see Appendix A for a full set of typedef fragments defined in
this document and the necessary code license.
4. Requirements for Mechanism Designers
All future GSS-API mechanism specifications MUST:
o list the set of GSS-API mechanism attributes associated with them.
5. IANA Considerations
The namespace of programming-language symbols with names beginning
with GSS_C_MA_* is reserved for allocation by IETF Consensus. IANA
allocated a base OID, as an arc of 1.3.6.1.5.5, for the set of
GSS_C_MA_* described herein, and registered all of the GSS_C_MA_*
values described in Section 3.2.
6. Security Considerations
This document specifies extensions to a security-related API. It
imposes new requirements on future GSS-API mechanisms, and the
specifications of future protocols that use the GSS-API should make
reference to this document where applicable. The ability to inquire
about specific properties of mechanisms should improve security.
The semantics of each mechanism attribute may include a security
component.
Application developers must understand that mechanism attributes may
be added at any time -- both to the set of known mechanism attributes
as well as to existing mechanisms' sets of supported mechanism
attributes. Therefore, application developers using the APIs
described herein must understand what mechanism attributes their
applications depend critically on, and must use the mechanism
attribute criticality features of these APIs.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000.
7.2. Informative References
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2025] Adams, C., "The Simple Public-Key GSS-API Mechanism
(SPKM)", RFC 2025, October 1996.
[RFC2847] Eisler, M., "LIPKEY - A Low Infrastructure Public Key
Mechanism Using SPKM", RFC 2847, June 2000.
[RFC4178] Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
Simple and Protected Generic Security Service Application
Program Interface (GSS-API) Negotiation Mechanism",
RFC 4178, October 2005.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, May 2006.
Appendix A. Typedefs and C Bindings
This appendix contains the full set of code fragments defined in this
document.
Copyright (c) 2009 IETF Trust and the persons identified as authors
of the code. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
- Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the
distribution.
- Neither the name of Internet Society, IETF or IETF Trust, nor the
names of specific contributors, may be used to endorse or promote
products derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
typedef const gss_buffer_desc * gss_const_buffer_t;
typedef const struct gss_channel_bindings_struct *
gss_const_channel_bindings_t;
typedef const <platform-specific> gss_const_ctx_id_t;
typedef const <platform-specific> gss_const_cred_id_t;
typedef const <platform-specific> gss_const_name_t;
typedef const gss_OID_desc * gss_const_OID;
typedef const gss_OID_set_desc * gss_const_OID_set;
#define GSS_S_BAD_MECH_ATTR (19ul << GSS_C_ROUTINE_ERROR_OFFSET)
OM_uint32 gss_indicate_mechs_by_attrs(
OM_uint32 *minor_status,
gss_const_OID_set desired_mech_attrs,
gss_const_OID_set except_mech_attrs,
gss_const_OID_set critical_mech_attrs,
gss_OID_set *mechs);
OM_uint32 gss_inquire_attrs_for_mech(
OM_uint32 *minor_status,
gss_const_OID mech,
gss_OID_set *mech_attrs,
gss_OID_set *known_mech_attrs);
OM_uint32 gss_display_mech_attr(
OM_uint32 *minor_status,
gss_const_OID mech_attr,
gss_buffer_t name,
gss_buffer_t short_desc,
gss_buffer_t long_desc);
Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
EMail: Nicolas.Williams@sun.com
AI ELI5
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PROBLEMGSS-API applications have no way to ask what properties a security mechanism has, so many hardcode mechanism object identifiers (OIDs).
MECHANISMMechanism properties become attributes named by OIDs under 1.3.6.1.5.5.13, and three new optional functions find mechanisms by attributes, list a mechanism's attributes and describe an attribute.
WHY IT MATTERSApplications can select or exclude mechanisms by capability, and future mechanism specifications MUST list their attributes.
§1The Hardcoded OID Problem
The Generic Security Services API (GSS-API) lets applications use different security mechanisms, each named by an object identifier (OID). Because there was no API to inquire about a mechanism's properties, many applications had to hardcode OIDs. With new mechanisms likely to appear, that does not scale.
The RFC's example is Secure Shell version 2, which supports GSS-API mechanisms for authentication (RFC 4462) but prohibits SPNEGO (RFC 4178). Future mechanisms that negotiate other mechanisms would need to be forbidden too, yet an application has no way to list mechanisms and programmatically exclude any 'like SPNEGO'.
Today a program has to know the exact name of each security tool. This RFC lets it ask what a tool can do instead.
§3.1Mechanism Attributes and Attribute Sets
A mechanism attribute describes a feature of a mechanism or of a pseudo-mechanism, which is a mechanism that relies on other mechanisms to provide its service. SPNEGO is the example: without other mechanisms it is useless.
To keep the interfaces simple, attributes reuse the existing OID and OID set types. The RFC defines an open namespace of attributes as arcs under 1.3.6.1.5.5.13. Each mechanism has a set of attributes it supports, as described in its specification.
An attribute is a label like 'waterproof' or 'has a zipper'. You can shop by labels without knowing the product name.
§3.2The Known Attributes
Section 3.2 lists 27 attributes, numbered as arcs 1 to 27, with arc 28 onward reserved. They fall into a few groups.
- Kind of mechanism: concrete, pseudo, composite, mechanism-negotiating (
GSS_C_MA_MECH_NEGO), glue (the GSS-API itself) and not-a-mechanism. - Default use:
GSS_C_MA_DEPRECATEDandGSS_C_MA_NOT_DFLT_MECHboth say the mechanism MUST NOT be used as a default. - Authentication: initiator-to-acceptor and acceptor-to-initiator, 'initial' authentication (passwords or keys on tokens) and anonymous names.
- Per-message protection: integrity, confidentiality, MIC and wrap tokens, protection before the context is fully established, replay detection and out-of-sequence detection.
- Other features: credential delegation, channel bindings, perfect forward security, compression and security context export/import.
The RFC then gives attribute sets for existing mechanisms. Kerberos V (RFC 1964) is concrete, offers mutual authentication, delegation, integrity, confidentiality and replay detection, among others, and its deprecated OID also carries GSS_C_MA_DEPRECATED. SPNEGO has just the negotiation and initial-token-framed attributes. SPKM attributes are left to a separate document.
The list is a big checklist of things a security tool might do, like checking who you are, scrambling messages, or spotting copies of old messages.
§3.4The Three New Functions
All three interfaces are OPTIONAL. Applications should use GSS_Indicate_mechs_by_attrs() instead of GSS_Indicate_mechs() wherever possible.
GSS_Indicate_mechs_by_attrs()takes three OID sets:desired_mech_attrs(the mechanisms MUST offer them),except_mech_attrs(they MUST NOT offer them) andcritical_mech_attrs(they MUST understand them). It returns the matching mechanisms. With all three empty it behaves likeGSS_Indicate_mechs()and returns every supported mechanism.GSS_Inquire_attrs_for_mech()takes a mechanism OID and returnsmech_attrs, the attributes it supports, andknown_mech_attrs, the attributes its implementation knows about. It returnsGSS_S_BAD_MECHfor an unknown mechanism.GSS_Display_mech_attr()takes an attribute OID and returns its name, a short description and a long description for humans.
A new major status, GSS_S_BAD_MECH_ATTR, reports an unknown attribute OID. In the C bindings it has routine error number 19, shifted by GSS_C_ROUTINE_ERROR_OFFSET. The C bindings also define new const typedefs, because the bindings in RFC 2744 apply const to pointer typedefs in a way that makes the pointer, not the pointed-to object, constant.
One function says 'find me tools that can do X but not Y'. Another says 'what can this tool do?'. A third says 'explain this label in words'.
- desired_mech_attrsAttributes the returned mechanisms MUST offer.
- except_mech_attrsAttributes the returned mechanisms MUST NOT offer.
- critical_mech_attrsAttributes the returned mechanisms MUST understand.
C prototypes
OM_uint32 gss_indicate_mechs_by_attrs(
OM_uint32 *minor_status,
gss_const_OID_set desired_mech_attrs,
gss_const_OID_set except_mech_attrs,
gss_const_OID_set critical_mech_attrs,
gss_OID_set *mechs);
OM_uint32 gss_inquire_attrs_for_mech(
OM_uint32 *minor_status,
gss_const_OID mech,
gss_OID_set *mech_attrs,
gss_OID_set *known_mech_attrs);
OM_uint32 gss_display_mech_attr(
OM_uint32 *minor_status,
gss_const_OID mech_attr,
gss_buffer_t name,
gss_buffer_t short_desc,
gss_buffer_t long_desc);§3.4.1Attribute Criticality
Attributes can be added at any time, and so can the attributes a mechanism supports. For example, a new attribute about contacting online infrastructure could be added, and Kerberos V would gain it without being modified. An application that relies on an attribute defined later cannot tell whether a given implementation knows about it.
The fix is the notion of critical attributes. An application lists the attributes it depends on as critical, and only mechanisms that understand them (meaning they know whether each is supported) are returned. The application can also ask which attributes a mechanism understands, through known_mech_attrs.
If you really need a tool to be waterproof, you only want tools whose makers actually checked the question, not ones that never heard of it.
§6Requirements, IANA and Security
All future GSS-API mechanism specifications MUST list the mechanism attributes associated with them. The GSS_C_MA_* symbol namespace is reserved for allocation by IETF Consensus, and IANA allocated the base OID as an arc of 1.3.6.1.5.5 and registered the values in Section 3.2.
The RFC says that being able to inquire about mechanism properties should improve security. Because attributes can be added at any time, developers must know which attributes their application depends on critically and MUST use the criticality features of these APIs. Each attribute's meaning may include a security component, and future protocols that use the GSS-API should refer to this document where it applies.
New tools must come with a label list. And if your program really needs a certain feature, it should say so, so it does not get a tool that merely never mentions it.
§7References
[RFC2743]RFC 2743Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000.January 2000[RFC2744]RFC 2744Wray, J., "Generic Security Service API Version 2 : C-bindings", RFC 2744, January 2000.January 2000[RFC1964]RFC 1964Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996.June 1996[RFC4178]RFC 4178Zhu, L., et al., "The Simple and Protected GSS-API Negotiation Mechanism", RFC 4178, October 2005.October 2005[RFC4462]RFC 4462Hutzelman, J., et al., "GSS-API Authentication and Key Exchange for the Secure Shell (SSH) Protocol", RFC 4462, May 2006.May 2006[RFC2847]RFC 2847Eisler, M., "LIPKEY - A Low Infrastructure Public Key Mechanism Using SPKM", RFC 2847, June 2000.June 2000